A Guide to Safe Electronic Weighing in Hazardous Locations

A practical guide to selecting weighing equipment for areas where flammable gas, combustible dust, or ignitable fibers may be present. It covers how a hazardous area is classified, the five recognized methods of protection, what each part of a weigh system needs, and how North American and international rules line up.

Why Weighing in a Hazardous Location is Different

In ordinary plants an electronic weigh system is simply a precise, reliable way to know how much material sits in a vessel. Where flammable gas, combustible dust, or ignitable fibers can be present, that same equipment also has to be incapable of becoming an ignition source. In the United States this is governed by the National Electric Code (ANSI/NFPA 70), whose stated aim is the practical safeguarding of persons and property from hazards arising from the use of electricity.

Articles 500 through 517 of the code cover the installation of electrical equipment in locations where fire or explosion hazards may exist. This guide condenses those rules as they apply to weighing equipment only; the physical installation itself remains the responsibility of the installing electrician or the engineering design firm. The single most important principle is also the simplest: wherever possible, keep electrical equipment outside the hazardous area entirely. Only when that is impractical do the methods of protection below come into play.

How a hazardous area is classified

A location is described along three independent axes: what kind of hazard is present, how likely it is to be present, and which specific substance it is. Together they decide how rugged the protection has to be.

• Class and Division Class sets the hazard type — Class I for gases and vapors, Class II for dust, Class III for fibers and flyings. Division sets the likelihood: Division 1 means the hazard is present in normal operation, Division 2 only under fault conditions.
• Groups and temperature Groups name the substance — A through D cover gases such as acetylene, hydrogen, ethylene, and propane; E through G cover metal, carbon, and grain dusts. A temperature code from T1 (450 °C) down to T6 (85 °C) marks the hottest surface allowed against the material's auto-ignition temperature.

Enclosure ratings sit alongside the classification

NEMA enclosure ratings describe what an enclosure keeps out. For hazardous areas the relevant ones are NEMA 7 (explosion-proof, Class I), NEMA 8 (oil-immersed contacts), NEMA 9 (dust-tight, Class II), and NEMA 10 (mining). The international equivalent is the two-digit IP code described further down. An enclosure rating and an area classification are separate things: the classification tells you how much protection the location demands, while the rating tells you what a given enclosure actually provides.

How Hazardous Areas are Classified

North American practice uses Classes and Divisions; most of the rest of the world uses IEC Zones. The two map onto each other closely, which matters when equipment crosses borders.

Zone 0 is the most demanding case — an explosive atmosphere present continuously — and the only equipment approved for it is intrinsically safe.

Five Methods of Protection

When equipment must sit inside the hazardous area, one of these methods makes it safe. The right choice is usually a balance of cost, simplicity, and reliability, and some systems combine more than one. They are listed from the lightest touch to the most rigorous; open any row for how it works, where it fits, and its trade-offs.

Protecting Each Part of a Weigh System

A single system can span several locations at once: sensors in a Division 1 area while the display sits in a safe room. Each part has its own energy level and its own route to compliance.

• Part 1 
  Load cells
  Strain gages count as simple apparatus. At 350 or 700 Ω and 10 to 15 Vdc a single 350 Ω cell draws at most 42.8 mA, making it non-incendive for Division 2. With no internal fault protection, Division 1 requires I.S. barriers.
• Part 2
  Summing units
  Resistive only, but up to eight 350 Ω cells in parallel can carry up to 350 mA at 15 Vdc. With no make-or-break contacts they are non-incendive for Division 2; Division 1 still needs I.S. barriers on the cells.
• Part 3
  Indicators & transmitters
  Microprocessor units run far below their supply voltage, around 5 V at under 1 mA at the switches. Suitable for Class I Division 2 as-is, Class II Division 2 in dust-tight enclosures, and Division 1 in explosion-proof or purged housings.
• Part 4
  I.S. barriers
  Mounted at the boundary between safe and hazardous areas. A fuse, a resistor, and a Zener shunt pass the measurement signal cleanly but clamp the voltage and current entering the hazardous area if a fault occurs.

Typical System Configurations

There is an almost unlimited range of hybrid layouts. These three show how the protection follows the location of each component and the division it sits in.

• Config 01 · Division 2
  Non-incendive system
  Load cells, the summing junction box, and the instrument are all rated for Class I and II, Division 2, Groups A–G. Because every part is non-incendive, no intrinsic safety barriers are needed.
  Result: the simplest compliant layout for a Division 2 area
• Config 02 · Division 1
  I.S. barriers, instrument in the safe area
  Intrinsically safe load cells and an I.S. summing box sit in the Division 1 area, with safety barriers at the boundary and the instrument located in the ordinary location.
  Result: full Division 1 protection with no explosion-proof instrument housing
• Config 03 · Division 1
  Purge or explosion-proof enclosures
  Load cells are protected by I.S. barriers while the summing unit and instrument sit in explosion-proof enclosures or under a Type Y purge, allowing the instrument to stay in the hazardous area.
  Result: Division 1 coverage where the instrument cannot be relocated

International and European Requirements

Outside North America, the IEC and CENELEC play roles similar to NFPA, FM, and NEMA, and their approach broadly agrees with the North American authorities.

The IEC sets uniform standards to encourage international trade; its explosion and ignition-protection principles, summarized in IEC Publication 79, were built on Articles 500 to 503 of the NEC. Instead of Classes and Divisions it uses Zones, which map closely onto the NEC scheme as shown in the table above.

Within Europe, CENELEC harmonizes hazardous-location standards as EN (Euronorm) standards based on the IEC recommendations. Each protection method carries a letter code: oil immersion "o" (EN 50015), pressurization "p" (EN 50016), sand filling "q" (EN 50017), flameproof "d" (EN 50018), increased safety "e" (EN 50019), and intrinsic safety "i" (EN 50020), all under the general requirements of EN 50014.

Enclosure protection abroad is stated as an IP (Ingress Protection) code: two digits after "IP" where the first, 0 to 6, rates protection against solid objects and dust, and the second, 0 to 8, rates protection against water. An IP65 enclosure, for example, is dust-tight and protected against water jets, the rough international counterpart to a NEMA rating.

The Complete Handbook, Free to Download

The full TC0013 handbook includes the full classification and temperature tables, the ignition curve, wiring diagrams, and the list of FM- and CSA-approved BLH Nobel products. For help matching a system to your area classification, talk to our application team.